3GPP standardisation body has a work item to define a solution for communication service identifiers in IMS (IP multimedia core network subsystem). The current versions of the related standards are 3GPP TS 24.229 v7.6.0 (2006-12) and 3GPP TS 23.228 v7.6.0 (2006-12). Communication service identifier work aims to provide a mechanism for the following issues: how to route SIP (Session Initiation Protocol) sessions to a certain AS (Application Server), how to charge the sessions, how to apply different policies to the session, and how to select the right application in a terminating UE (User Equipment) on the basis of a certain service identifier value. Examples of such service identifier values are OMA-POC (Open Mobile Alliance—push to talk over cellular), OMA-IM (IP multimedia) and 3GPP MMTel (multimedia telephony service).
The following has been proposed for the communication service identifiers in change request 24.229 CR 1579, rev 3, “Clarification of the handling ICSI”: An originating UE that supports and wishes to initiate a certain service adds the corresponding service identifier value to an Accept-Contact header of an outgoing request. Only one value can be added per request. In addition, the originating UE may add this value and also all other supported service identifier values into a Contact header. A terminating UE should then determine the received service identifier value in the Accept-Contact header and select an application to be used accordingly. In addition, the terminating UE should add this value and also all other supported service identifier values to the Contact header of a first reliable or final response sent to the originating UE.
Certain rules for an UE to understand in connection with session establishment, whether the session supports a particular service, have been proposed. Namely, if a terminating UE receives a service identifier value in the Accept-Contact header, or an originating UE receives in the Contact header of a response the same service identifier value it included in the Accept-Contact header of the outgoing request, then the UE understands that the session is able to support the service.
In 3GPP IMS service architecture, entities called B2BUAs (back-to-back user agent) implement the services in the network. Currently it has been proposed that B2BUAs between UEs may replace or add the communication service identifiers in the Accept-Contact and Contact headers. This in combination with the currently proposed rules for UEs to interpret the content of the Accept-Contact and Contact headers results in that UEs cannot know for sure whether some service identifier has been added by a B2BUA or the other UE. For this reason the UEs cannot know, whether the end-to-end session really supports the requested service or not. That is, the currently proposed solution does not work properly.
Thus there is a need for further consideration.